Instant and sensitive detection of pathogens (e.g., bacteria, viruses) at ultra-low concentrations without time consuming procedures, such as incubation or amplification by polymerase chain reaction (PCR) offers obvious clinical benefits. However, there are, to date, no general and satisfactory assays that can detect bacteria at concentrations of <102 colony forming units per milliliter (cfu/mL) without pre-enriching the bacteria via a culture process. 1 
Compared to magnetic beads (1-5 μm in diameter) used in biological separations, magnetic nanoparticles2,3 (less than 10 nm in diameter) promise high-performance because of their large surface/volume ratios and easy entry into cells. 4 Despite intense efforts in the study of magnetic nanoparticles, the application of magnetic nanoparticles in biomedicine is just emerging. 5 